The invention relates to a control for a drive motor in a sewing machine. More particularly, this invention relates to a control system which will regulate motor speed in a stable fashion as such speed is increased and decreased and which will also stop the motor.
A prior art system of this kind is shown in FIG. 1, in which together with thyristors 2, diodes 3, form a full-wave rectifying bridge which supplies a DC motor. Thyristors 2 are phase-controlled by a gate control device (not shown) connected to their gates G.sub.2, and the motor 4 is thus speed-controlled by the phase-controlled voltages applied to gates G.sub.2, and numerals 7 and 8 are free wheel diodes. A braking thyristor 9 is connected in parallel with the armature 5, and after receiving a control signal (not shown) at its gate G.sub.9, short-circuits an armature 5 during motor braking, so as to consume the motor's rotational energy by dissapating it in the armature's own resistance. FIG. 2 shows the operating relations of the circuit elements during the drive and braking periods of the motor. In FIG. 2 line G.sub.2 designates a presence or non-presence of a gate signal of motor control thyristors 2, and during the period t.sub.1 to t.sub.2 indicates a phase-control voltage applied to thyristors 2. Line G.sub.9 shows a gate signal applied to braking thyristor 9, and the period between t.sub.2 and t.sub.4 shows a period of time in which the thyristor 9 is conductive (as shown by the level of line 9) to brake motor 4. Line N shows the speed of the motor 4, which during the period between t.sub.1 and t.sub.2 is driven at a speed determined by the gate signal G.sub.2. When thyristor 9 is conductive at the point t.sub.2 for braking the motor, field coil 6 is energized via short-circuited thyristor 9, since the thyristor 2 has been fired. Armature 5, therefore, generates reverse EMF from time t.sub.2 and the armature 5 carries current via thyristor 9. The rotational energy of the armature and the elements connected thereto is then consumed by the internal resistance of armature 5 and motor 4 is rapidly stopped. In FIG. 2, motor speed N is reduced at time t.sub.2, following the solid curved line. But, depending upon AC power source phase, speed, and motor load at time t.sub.2, the required stopping time for the motor can vary as is shown by dotted lines a and b. Therefore, in order to exactly stop the motor in a predetermined period, gate signals G.sub.2 and G.sub. 9 are prolonged respectively, to times t.sub.3 and t.sub.4. Accordingly, the field coil 6 is unnecessarily heated by a comparatively large electric current until time t.sub.3 --i.e. even after the motor stops. The prolongation of gate signal G.sub.9 serves the purpose of preventing the motor from rerotating. Such rerotation otherwise take place because cutoff of thyristor 2 is unstably dependent upon the phase of the AC power source at time t.sub.3, and may lag about a half cycle behind time t.sub.3, causing field coil 6 to be energized after cutoff of thyristor 9. This is the reason why such a timing control is necessary. However, such a timing control may cause erroneous operations of the circuit elements.
The present invention has been provided to eliminate such defects and disadvantages of the prior art.